A transformer has become an essential electronic component for voltage regulation into required voltages for various kinds of electric appliances.
FIG. 1 is a schematic exploded view illustrating a conventional transformer. As shown in FIG. 1, the transformer 1 comprises a magnetic core assembly 11, a first bobbin piece 12, a second bobbin piece 13, a primary winding coil 14 and a secondary winding coil 15. The first bobbin piece 11 has a first side plate 16. The second bobbin piece 13 has a second side plate 17 and plural partition plates 13a. Several winding sections 13b are defined by every two adjacent partition plates 13a. According to a voltage-dividing principle, the number of winding sections 13b may be varied depending on the voltage magnitude. In addition, a first base 16a and a second base 17a are extended from the first side plate 16 and the second side plate 17, respectively. Several pins 18 and 19 are respectively arranged on the bottom surfaces of the first base 16a and the second base 17a. 
For winding the primary winding coil 14 on the first bobbin piece 11, a first terminal of the primary winding coil 14 is firstly wound around and fixed on a pin 18a under the first base 16a. The primary winding coil 14 is then successively wound on the first bobbin piece 11 in the direction distant from the first side plate 16. Then, the primary winding coil 14 is returned back, and a second terminal of the primary winding coil 14 is wound around and fixed on another pin 18b under the first base 16a. For winding the secondary winding coil 15 on the second bobbin piece 13, a first terminal of the secondary winding coil 15 is firstly wound around and fixed on a pin 19a under the second base 17a. The secondary winding coil 15 is then successively wound on the winding sections 13b of the second bobbin piece 13 in the direction distant from the second side plate 17. Then, the secondary winding coil 15 is returned back, and a second terminal of the secondary winding coil 15 is returned to be wound around and fixed on another pin 19b under the second base 17a. Moreover, due to the partition plate 13a of the second bobbin piece 13, the primary winding coil 14 is separated from the secondary winding coil 15, thereby maintaining an electrical safety distance and increasing leakage inductance of the transformer 1.
The winding structure of the transformer 1, however, still has some drawbacks. For example, since the primary winding coil 14 and the secondary winding coil 15 are returned back to be respectively fixed on the pins 18b and 19b under the first base 16a and the second base 17a, portions of the primary winding coil 14 and the secondary winding coil 15 are exposed under the first bobbin piece 11 and the second bobbin piece 13. Even if the exposed portions are covered by insulating material, the safety distance is also insufficient. Under this circumstance, the transformer 1 is readily suffered from high-voltage spark or short circuit and eventually has a breakdown.
For increasing the working voltage, two or more transformers are connected in parallel to collectively define a transformer set. FIG. 2 is an exploded view illustrating a transformer set according to the prior art. As shown in FIG. 2, the transformer set comprises a first transformer 2 and a second transformer 3, which are arranged in parallel with each other. The first transformer 2 comprises a first bobbin piece 22 and a second bobbin piece 23. The second transformer 3 also comprises a first bobbin piece 32 and a second bobbin piece 33. The first bobbin pieces 22 and 32 have the same structures. The second bobbin pieces 23 and 33 have the same structures. Since the first transformer 2 and the second transformer 3 have the same structures, only the first transformer 2 is illustrated for clarification herein.
The first bobbin piece 22 comprises a base 221, a sheathing part 222 and a first channel 223. A winding section 224 is arranged between the base 221 and the sheathing part 222 for winding a primary winding coil 24. The sheathing part 222 is substantially a rectangular hollow structure with a receptacle 222a. The first channel 223 runs through the base 221 and the winding section 224. In addition, the first channel 223 is in communication with the receptacle 222a of the sheathing part 222. The second bobbin piece 23 and the secondary winding coil 25 that is wound around the second bobbin piece 23 are accommodated within the receptacle 222a of the sheathing part 222. As such, by the sheathing part 222, the primary winding coil 24 and the secondary winding coil 25 are separated from each other, and the first channel 223 of the first bobbin piece 22 and the second channel 235 of the second bobbin piece 23 are in communication with each other. The second bobbin piece 23 comprises a first side plate 230, a second side plate 238, plural partition plates 232, a slab 233 and a base 231. The first side plate 230, the second side plate 238, the partition plates 232 and the base 231 have rectangular profiles. The first side plate 230 and the second side plate 238 are arranged on two opposite sides of the second bobbin piece 23. In addition, the first side plate 230 and the second side plate 238 are perforated plates. The partition plates 232 are parallel with the first side plate 230 and the second side plate 238. As such, the partition plates 232, the first side plate 230, the second side plate 238 and the slab 233 collectively define plural winding sections 234 for winding the secondary winding coil 25.
The base 231 is extended from the first side plate 230 and connected with the first side plate 230. In addition, a first pin 236 and a second pin 237 are disposed on the base 231 to be inserted into corresponding holes of a circuit board (not shown). The first pin 236 and the second pin 237 are L-shaped. The first pin 236 comprises a first coupling part 236a and a second coupling part 236b. The second pin 237 comprises a first coupling part 237a and a second coupling part 237b. For winding the secondary winding coil 25 on the second bobbin piece 23, a first terminal of the secondary winding coil 25 is firstly wound around and fixed on the first coupling part 236a of the first pin 236. The secondary winding coil 25 is then successively wound on the winding sections 234, and then wound on an auxiliary part 239. Then, the secondary winding coil 25 is returned back, and a second terminal of the secondary winding coil 25 is fixed onto the first coupling part 237a of the second pin 237.
Please refer to FIG. 2 again. The transformer set further comprises a magnetic core assembly 31. The magnetic core assembly 31 comprises a first magnetic part 311 and a second magnetic part 312. The first magnetic part 311 includes a first lateral leg 311a and a second lateral leg 311b. The second magnetic part 312 includes a first lateral leg 312a and a second lateral leg 312b. For connecting the first transformer 2 and the second transformer 3 in parallel, the first lateral leg 311a and the second lateral leg 311b of the first magnetic part 311 are respectively embedded into the first channel 223 of the transformer 2 and the first channel 323 of the transformer 3, and the first lateral leg 312a and the second lateral leg 312b of the second magnetic part 312 are respectively embedded into the second channel 235 of the transformer 2 and the second channel 335 of the transformer 3. As such, the transformers 2 and 3 are combined together to produce the transformer set. The transformer set, however, still has some drawbacks. For example, after the both terminals of the secondary winding coil 25 are respectively fixed on the first coupling parts 236a and 237a of the pins 236 and 237, the L-shaped pins 236 and 237 are readily shifted. In this situation, the terminals of the secondary winding coil 25 are possibly broken. As known, the increase of wire diameter of the secondary winding coil 25 may reduce the possibility of breaking the terminals of the secondary winding coil 25. However, since the wire diameter is increased, the bobbin pieces need to be re-designed and re-produced. The process of re-designing and re-producing the new bobbin pieces is time-consuming and costly. Moreover, since the bobbin pieces 22, 23, 32 and 33 are made of plastic material, the bases 221, 231, 321 and 331 are readily subject to deformation during the fabricating process because the structural strength is usually insufficient.
Moreover, since the auxiliary part 239 fails to facilitate fixing the returning portion of the secondary winding coil 25 at a specified position, the secondary winding coil 25 is readily abraded when the second bobbin piece 23 is accommodated within the sheathing part 222 of the first bobbin piece 22. In a case that the distance between the returning portion of the secondary winding coil 25 and the secondary winding coil 25 wound on the winding section 234 is very short, the transformer 2 is readily suffered from high-voltage spark. Even if the distance between the returning portion of the secondary winding coil 25 and the secondary winding coil 25 wound on the winding section 234 is increased, the returning portion of the secondary winding coil 25 becomes close to the secondary winding coil 35, and thus the possibility of causing high-voltage spark is increased. In this circumstance, the transformers 2 and 3 are readily damaged.
Therefore, there is a need of providing an improved transformer set so as to obviate the drawbacks encountered from the prior art.